(Not Applicable)
(Not Applicable)
The present invention pertains generally to fluid flow control and, more particularly, to a control system for a pneumatic circuit with improved cylinder sensitivity effected through the use of check valves for selectively isolating components on either end of the cylinder.
Pneumatic systems typically involve a source of compressed air that is routed through a network of pipes. The compressed air is typically obtained from a compressor which is usually driven by an electric motor or an internal combustion engine. The compressed air is routed to a positioner which ultimately controls the flow of compressed air to and from a cylinder in order to move a piston sealed within the cylinder. The piston may have a shaft extending out of the cylinder and connected to the component to be moved. The positioner provides pneumatic signals in the form of compressed air which is routed to control valves or boosters. The boosters are selectively opened and closed to regulate the flow of the compressed air to and from the cylinder. The boosters receive the pneumatic signals and may be opened and closed by pneumatic pilots connected on either end of each booster. The pneumatic pilots of the boosters are connected to the positioner through signal lines. The boosters are also connected to the source of compressed air through feed lines. The signal lines are typically of a smaller diameter than feed lines because they supply and exhaust compressed air into and out of the cylinder at relatively low flow rates. However, at higher flow rates, the positioner provides a greater flow of compressed air into the signal lines with a pressure sufficient to actuate the pneumatic pilots of the volume boosters. The actuated boosters allow compressed air to flow from the larger diameter feed lines into and out of the cylinder at the higher rate.
The pneumatic system moves the piston by forcing air into a first end of the cylinder while simultaneously withdrawing or exhausting air out of a second end of the cylinder in order to advance the piston along the length of the cylinder. Conversely, the pneumatic system may also force air into the second end of the cylinder while simultaneously exhausting air out of the first end of the cylinder in order to retract the piston in the opposite direction. By driving the air into alternate ends of the cylinder, the piston is moved such that the shaft can be displaced in any position for doing useful work. The compressed air may pass through a regulator to control the amount of pressure available in the pneumatic circuit. The compressed air may also pass through a filter to clean the air to prevent damage to components thereby ensuring that the components have a long and reliable working life.
Pneumatic systems are commonly used in large scale applications such as in power plants and refineries for controlling system components such as a working valve. In such applications, it may be desirable to quickly and repeatedly position the piston to within thousandths of an inch. In order to quickly and precisely position the piston, a pair of boosters may be connected to the first end of the cylinder arid another pair of boosters may be connected to the second end of the cylinder. The first pair of boosters may include one booster configured for xe2x80x9cpushingxe2x80x9d air into the first end of the cylinder with the other of the pair configured to xe2x80x9cpullxe2x80x9d air from the first end. Likewise, the second pair may include one xe2x80x9cpusherxe2x80x9d and one xe2x80x9cpullerxe2x80x9d booster. Quick exhaust valves may also be installed between the puller boosters and the respective first and second ends of the cylinder. Operating in conjunction with the puller boosters, the quick exhaust valves exhaust air out of the cylinder at high flow rates. Although configured to respectively supply and exhaust air into and out of the cylinder at high flow rates, the pusher and puller boosters also have the individual capability to respectively exhaust and supply air out of and into the cylinder, although at significantly lower flow rates. At low flow rates, the pusher boosters on the first and second ends of the cylinder supply compressed air to the cylinder solely through the smaller diameter signal lines. However, at higher flow rates, the positioner provides sufficient pressure of compressed air to the pneumatic pilots through the signal lines such that the pusher boosters are actuated. Depending on whether the compressed air is to be supplied to the first end or to the second end of the cylinder, the first or second actuated pusher booster will allow compressed air to flow to the first or second end through the larger diameter feed lines. For example, if compressed air is to be supplied to the first end at a high flow rate, then the pusher booster connected to the first end provides the majority of compressed air to the first end while the puller booster connected to the first end provides a negligible amount of compressed air. Simultaneously, the puller booster connected to the second end exhausts the majority of compressed air from the second end while the pusher booster connected to the second end exhausts a negligible amount of compressed air.
The sensitivity of the boosters in responding to pneumatic signals is controlled by adjustable restrictions or needle valves which are incorporated into the boosters. The needle valves are connected in parallel across the boosters at the pneumatic pilots. When the pressure of compressed air acting on the pneumatic pilots reaches a preset level, the booster toggles from a xe2x80x9cclosedxe2x80x9d or null position to a supply or exhaust position. In either the supply or exhaust position, a greater flow of compressed air from feed lines may pass through the boosters and enter or exit alternate ends of the cylinder. Thus, the adjustable restrictions provide a means for setting the point at which the booster are activated by the pneumatic pilots so that the booster toggles from the null position to either the supply or the exhaust position.
As mentioned above, the positioner adjusts the position of the piston by forcing air into alternate ends of the cylinder. However, due to the compressible nature of air, dynamic instability may result within the pneumatic circuit such that the piston is difficult to precisely and rapidly position. For example, within typical pneumatic circuits, when there are active components such as a pusher and a puller booster connected to a first end of the cylinder, the adjustment of the sensitivity of the pusher booster may affect the total capacity of the compressed air into the cylinder on that same first end of the cylinder. More specifically, in the example, if the sensitivity of the pusher booster in responding to pneumatic signals is increased, the pusher booster will toggle to the supply position in response to relatively small pneumatic signal changes. However, the non-activated puller booster will simultaneously provide a small flow of compressed air to the cylinder through the signal lines. Because of the compressibility of air, the piston will not start to move toward the second end until both the pusher and puller booster on the first end have sufficiently pressurized. Thus, the overall speed of the piston in responding to signal changes is reduced. In addition, the position of the piston within the cylinder may fluctuate as the boosters respond to small signal changes, resulting in dynamic instability. In addition, because the non-activated booster on either side of the cylinder must supply compressed air through signal lines each time the piston moves, the total requirement of compressed air that must be provided by the positioner to regulate the piston position is increased.
The prior art discloses several pneumatic circuits with control systems designed to improve the accuracy and response time with which the piston may be positioned within the cylinder. One such prior art device includes an actuator system which modulates a linear output shaft associated with a working control valve in response to a control signal input. The system includes a feedback control link, a pneumatically controlled hydraulic valving system and a hydraulic cylinder and piston controlled by the hydraulic valving system. The hydraulic valving system includes a three-position, four-way valve actuated by pneumatic binary output signals from a signal conditioner which is in turn controlled by the positioner. Hydraulic flow to the three-position, four-way valve may also be controlled from the signal conditioner in response to positioner output for effective actuation of the hydraulic piston and cylinder assembly. Although the system exhibits rapid response time and high accuracy in positioning the piston with n the cylinder, the system is necessarily complex and costly in that it combines hydraulic circuit components with pneumatic circuit components. Furthermore, the system disclosed in the reference is not easily retrofittable into existing pneumatic circuits.
As can be seen, there exists a need in the art for a pneumatic control system wherein the opening and closing speeds of the control valves or volume boosters can be adjusted with minimal impact on the overall speed of the piston within the cylinder. In addition, there exists a need in the art for a pneumatic control system wherein the total requirement of compressed air out of the positioner is minimized. Furthermore, there exists a need in the art for a pneumatic control system wherein the interactive effects of the volume boosters on the first and second ends of the cylinder may be eliminated. Finally, there exists a need in the art for a pneumatic control system that may be retrofitted into existing pneumatic circuits.
The present invention specifically addresses and alleviates the above referenced deficiencies associated with pneumatic control systems. More particularly, the present invention is an improved pneumatic control system for positioning a piston within a cylinder of a pneumatic circuit. As will be demonstrated below, the pneumatic control system of the present invention differs from pneumatic control systems of the prior art in that it utilizes booster check valves for increasing the responsiveness of the pneumatic control system to pneumatic signals.
The pneumatic control system is configured for positioning a piston within a cylinder having first and second ends by manipulating a flow of compressed air such that the position of the piston may be regulated. A compressed air source provides compressed air to the pneumatic circuit. A filter regulator may be included in the pneumatic circuit to reduce the pressurization level of the source of air to a safe working level. The filter regulator also filters the source of compressed air to remove contaminates.
A positioner regulates the flow of compressed air into and out of the first and second ends of the cylinder. A piston position signal representative of an actual piston position may be supplied to the positioner. The positioner converts the piston position signal to a pneumatic signal representative of a desired piston position. In response to the pneumatic signal, the flow of compressed air may be alternately directed into the first and second ends for respectively retracting and extending the piston to correct for disparity between the actual piston position and the desired piston position.
A directional valve fluidly connected to the compressed air source includes a pneumatic pilot connected to the air source. When pressure in the signal line overcomes a biasing spring force, the directional valve opens such that compressed air may be delivered to one of the two signal lines exiting the directional valve. The directional valve may be set to close when the pressure of the compressed air drops below 50 psi, as a failsafe mechanism.
Importantly, first and second small and first and second large booster check valves are utilized to block the flow of compressed air in one direction of the pneumatic circuit and allow free flow of the compressed air in the opposite direction. The first and second small booster check valves are oriented such that compressed air flowing toward first and second small boosters may be blocked. Conversely, the flow orientations of the first and second large booster check valves are such that compressed air may flow only toward first and second large boosters. Advantageously, the first and second small booster check valves isolate the first and second small boosters such that compressed air is blocked from flowing into the first and second small boosters. Likewise, the first and second large booster check valves operate to isolate the first and second large boosters such that compressed air is blocked from flowing through the signal lines toward the directional valve. By selectively isolating the first and second small and large boosters, the total volume of compressed air that would otherwise flow into the respective first and second small and large boosters is reduced. By reducing the total amount of compressed air that is required in order to effect a given piston movement, the speed with which the boosters may be activated is reduced. The reduced total volume of compressed air that would otherwise be required to effect a given piston movement ultimately allows for more effective control of the piston within the cylinder.
The first small and large boosters are positioned on the first end of the cylinder for supplying and exhausting compressed air into and cut of the first end of the cylinder. The second small and large boosters are positioned on the second end of the cylinder for supplying and exhausting compressed air into and out of the second end of the cylinder. The first and second small and large boosters each include pneumatic pilots connected to the air source via signal lines for overcoming the spring bias to force the booster to either one of the two alternate positions. The boosters include adjustable restrictions for regulating the sensitivity of the boosters in responding to changes in pressure of the compressed air provided by the positioner through the signal lines.
First and second quick exhaust valves operate in conjunction with the first and second small boosters to quickly exhaust compressed air out of the respective first and second ends of the cylinder at high flow rates. At low flow rates, the first and second small boosters alternately exhaust air out of the second end unaided by the first and second quick exhaust valves. At high flow rates, the first and second quick exhaust valves are initiated by the increased pressure differential resulting from the initial low rate of alternate exhaustion of compressed air out of the first and second small boosters.
The cylinder is interposed between the first large booster and first quick exhaust valve and the second large booster and second quick exhaust valve at the respective first and second ends. Sealed within the cylinder is the piston. The piston is connected to a shaft which extends out of the cylinder, the shaft being connectable to a component to be moved.